Various types of angular position sensors are known. For example, as a target object rotates, one of those angular position sensors is adapted to rotate magnetic sensor means having two magnetic sensor elements relative to magnetic field generator means such as permanent magnets. Then, the angular position sensor detects an angle of rotation of the target object based on an output signal of the magnetic sensor elements which varies as the target object rotates (e.g., see Japanese Patent Laid-Open Publications Nos. 62-95402 and 60-47901).
An example of such an angular position sensor is shown in FIG. 8. The angular position sensor includes a pair of permanent magnets 312 and 314, which are attached to the inner circumferential wall of a cylindrical yoke 310 to form a parallel magnetic field. The yoke 310 and the permanent magnets 312 and 314 rotate in conjunction with a target object whose angle of rotation is to be detected. For example, provided generally at the center of the yoke 310 as the magnetic sensor element are Hall elements 320 and 322 which are supported by a support member to form an angle of 90 degrees to each other in the direction of rotation of the target object. That is, as shown in FIG. 9, output signals 100 and 102 from the Hall elements 320 and 322 are different in phase by 90 degrees from each other, the output signals having a sine and cosine relation.
Accordingly, as shown in FIG. 10, applying a trigonometric arc tangent operation to the output signals 100 and 102 shown in FIG. 9 results in a calculated angle 110 which varies in a cycle of 180 degrees in a one-to-one relation with the angle of rotation of the target object.
Then, as shown in FIG. 12, the rotational angular position of the target object is identified within a range from 0 to 360 degrees in accordance with the sign of the output signals 100 (Va) and 102 (Vb). Then, an offset angle is added to the calculated angle 110 of FIG. 10 to provide a combined calculated angle 110, thereby providing an output angle 120 which varies in a cycle of 360 degrees in a one-to-one relation with the angle of rotation of the target object, as shown in FIG. 11. This makes it possible to detect the angle of rotation of the target object within a range from 0 to 360 degrees.
However, in practice, like an output signal 130 shown in FIG. 13, an offset may be added to the output signal of a Hall element, so that the signal is shifted from the center of oscillation. Furthermore, some variations in gain of the output signals 130 and 132 may also be found between the Hall elements. These offset and variation in gain may be caused during manufacture of the Hall elements or installation of the angular position sensor.
The angle of rotation of the target object cannot be properly determined from output signals which have an offset or a variation in gain. It is thus necessary to make a correction to the offset and the gain of the output signals 130 and 132 in order to determine the angle of rotation, as described in Japanese Patent Laid-Open Publications Nos. 2001-311605 and 2004-45286.
As described in Japanese Patent Laid-Open Publications Nos. 2001-311605 and 2004-45286, a correction is made by detecting the minimum and maximum value of the output signals and then determining their offset and gain. However, for example, using Hall elements as the magnetic sensor element requires a rotation of 180 degrees or more of the permanent magnets or the like which form a magnetic field, thereby resulting in an increase in time for correcting the output signals.
Furthermore, suppose that the angular position sensor attached to a target object may degrade over time, so that the output signal from a Hall element needs to be corrected. In this case, for example, the range of rotational angles of the target object being as narrow as less than 90 degrees would also raise another problem that the minimum or maximum value of the output signal cannot be determined, thereby causing no correction to be made to the output signal.